An infant is a human child at the youngest stage of life.
Some people say that infancy is until someone can walk,
while say infancy is the time before the age of one. An
infant less than 1 month old is a newborn. Newborns can
be premature infants, postmature infants and full term
newborns.
After learning to walk, infants are called toddlers,
usually when they are 1-3 years old.
Source : Baby